1. Field of the Invention
The invention relates to a motor assembly for an electret transducer comprising a diaphragm plate and a backplate with spacing means between the diaphragm plate and the backplate for supporting the diaphragm plate on the backplate.
2. Prior Art
In the prior art, for instance as disclosed in U.S. Pat. No. 3,740,496, titled "Diaphragm Assembly for Electret Transducer", the central vibratable diaphragm plate and the backplate have protrusions or posts therebetween on which the diaphragm plate rests to thereby properly position the diaphragm plate relative to the electret which is mounted on the metal backplate. A practical difficulty arises in this prior art because the protrusions supporting the diaphragm plate are formed out of the backplate material by impressing a die on the opposite side thereof. With this technique it is a problem to form such protrusions within the stringent and close tolerances which are required to maintain and accurately reproduce the spacing between the diaphragm plate and the oppositely lying electret foil mounted on the backplate surface.
It is an object of the subject invention to provide a solution to the above problem. More in particular it is an object of the invention to provide a motor assembly for an electret transducer, which can be manufactured with a simple and economic process while meeting the stringent requirements imposed to the spacing between the diaphragm plate and backplate, and also having a high yield in a series of mass produced motor assemblies and transducers. It is a further object of the invention to improve the performance of an electret transducer thereby that the ratio between the total area of contact between the diaphragm plate and the diaphragm plate supporting protrusions and the freely movable area of the diaphragm plate is lowered compared to a similar ratio for a prior art structure.
Further it is an object of the invention to provide a method for manufacturing a motor assembly for an electret transducer and which method is appropriate for an economic production.